I. Field of the Invention
This invention relates to a power supply for driving an electronic load and more particularly to a DC power supply of the free-running flyback topology.
II. Description of the Prior Art
Off-line switching mode power supplies are commonly used in computers to achieve small size and high efficiency. The 115/230 volts (AC) input power is rectified and passed through a regulated DC-DC converter to produce the various voltages needed for logic networks, disk drives, etc. A type of regulating converter in common use is the flyback or ringingchoke type that stores energy in a multi-winding choke (power transformer) during the first half cycle and delivers this energy to the output load during the second half cycle. The storage and discharge portions of the duty cycle are generally controlled by a power transistor which acts as a switch. This transistor switch operates in the saturated and cut-off modes in response to a signal from a control transistor which is driven by an analog feedback signal.
A common form of flyback topology is the discontinuous flyback topology, in which the choke magnetic field is allowed to collapse completely after the second half cycle before a new cycle starts, leaving no energy stored in the choke. Two forms of discontinuous flyback topology are constant frequency, in which the cycle is started at regular fixed intervals, and free running, in which the new cycle starts immediately after the previous half cycle ends.